1. Field of the Invention
The present invention relates to a variable displacement pump for use as a hydraulic pressure supply source for an automobile power steering device, for example.
2. Description of the Related Art
A variable displacement pump according to the related art of this kind is well known in which the discharge flow rate is controlled by increasing or decreasing the volume of a pump chamber, as disclosed in JP-A-6-200883, for example. Referring to FIGS. 9 to 12, the variable displacement pump disclosed in this publication will be described below.
FIG. 9 is a cross-sectional view of a variable displacement pump according to the related art, taken perpendicularly to an axial line of a drive shaft. FIG. 10 is a cross-sectional view of the variable displacement pump according to the related art, taken along the axial line of the drive shaft. FIGS. 11 and 12 are cross-sectional views showing the constitution of a control valve and a discharge passage. In these figures, reference numeral 2 denotes a pump body of the variable displacement pump (indicated by numeral 1 as a whole), which has a front body 4 like a cup located on the left of FIG. 10 and a rear body 5 like a plate located on the right of FIG. 10.
The front body 4 has a circular concave portion 6 opening to the right of FIG. 10, in which the pump components including a pressure plate 7, a cam ring 8, a rotor 3 and an adapter ring 9 are inserted within this concave portion 6. A circular convex portion 5a formed on a front face of the rear body 5 is fitted into an opening portion of this front body 4, and the front body 4 and the read body 5 are secured by a securing bolt 10 to close the circular concave portion 6 of the front body 4. The circular convex portion 5a of the rear body 5 constitutes one side wall of the pump chamber 11 as will be described later, and owing to an O-ring 12 attached around an outer circumferential face thereof, a pressure oil is prevented from leaking out of the pump body 2.
The pressure plate 7 disposed on the bottom side of the circular convex portion 6 for the front body 4 has a circular plate portion 7a making up the other side wall of the pump chamber 11 and a cylindrical portion 7b formed in an axial core of this circular plate portion 7a, in which this circular plate portion 7a is fitted with the inner circumferential face of the circular concave portion 6 of the front body 4. An O-ring 13 is attached around the outer circumference of this circular plate portion 7a to prevent pressure oil from leaking through a gap between the circular plate portion 7a and the front body 4. The pressure plate 7 is disposed on the bottom face side of the circular concave portion 6 of the front body 4. The adapter ring 9 is fitted on the outer circumferential portion of the pressure plate 7. The cam ring 8 and the rotor 3 are contained inside this adapter ring 9.
The cam ring 8 acts to increase or decrease the pump volume of the variable displacement pump 1, and is carried by the adapter ring 9 to be swingable around a seal pin 14 provided on inner circumference of the adapter ring 9 and on the lower side in FIG. 9 as a swinging fulcrum. Also, the cam ring 8 is urged on the left of FIG. 9 by urging means 15. This urging means 15 has a plug 16 screwed into the front body 4, and a compression coil spring 17 resiliently attached between the plug 16 and the cam ring 8. This compression coil spring 17 is inserted through a through hole 9a formed in the adapter ring 9 to contact with the cam ring 8.
The cam ring 8 is swung reciprocatively by supplying pressure oil from a control valve 23 selectively to a first fluid pressure chamber 21 formed in one swing direction (on the left of FIG. 9) or a second fluid pressure chamber 22 formed in the others wing direction. The first fluid pressure chamber 21 and the second fluid pressure chamber 22 are partitioned with the seal pin 14 and a seal member 24 attached at a position in axial symmetry to the seal pin 14 of the cam ring 8. Sealing between both the fluid pressure chambers 21 and 22 is kept by the seal pin 14 and the seal member 24.
The rotor 2 disposed inside the cam ring 8 is connected to a drive shaft 25 having a motive power transmitted from an engine, not shown, and has a plurality of vanes 27 carried to be able to emerge from its outer circumference and sliding with an inner circumferential cam face of the cam ring 8. The drive shaft 25 for rotating the rotor 3 is rotatably supported via the bearings 28, 29 and 30 within the pump body 2. The rotor 3 is rotated by the drive shaft 25 in a counterclockwise direction (as indicated by the arrow) in FIG. 9.
This variable displacement pump 1 sucks a working oil from a suction pipe 31 and a suction passage 31a, which are fixed to the rear body 5, through a suction opening 32 formed in the convex portion 5a of the rear body 5 into the pump chamber 11, as shown in FIG. 10. Also, the working oil sucked into the pump chamber 11 is discharged through a discharge opening 33 formed in the circular plate portion 7a of the pressure plate 7 to a discharge pressure chamber 34 formed on the bottom of the front body 4. The discharge flow rate of this variable displacement pump 1 is at maximum in a state where the cam ring 8 is swung on the left as shown in FIG. 9 and decreases when the cam ring 8 is swung on the right of FIG. 9.
The discharge pressure chamber 34 is formed annularly between the outer circumference of the cylindrical portion 7b of the pressure plate 7 and the bottom face of the circular concave portion 6. The discharge passage 35 is connected to an upper portion of the discharge pressure chamber 34 in FIG. 10. A pressure oil discharged from the pump chamber 11 to the discharge pressure chamber 34 is fed through this discharge passage 35 to a power steering device PS. The discharge passage 35 has a radial portion 35a extending from the discharge pressure chamber 34 outwards in the radial direction of the rotor 3, and a transversal portion 35b extending in a direction orthogonal to this radial portion 35a, as shown in FIG. 10. A feed oil pipe (not shown) for feeding pressure oil to the power steering device PS is connected to an end portion of this transversal portion 35b. Also, the transversal portion 35b of the discharge passage 35 is provided with a metering orifice 36 (see FIG. 11).
The control valve 23 has a spool 38 fitted slidably within a valve bore 37 formed in the front body 4. The spool 38 partitions the inside of the valve bore 37 into the first to fourth oil chambers 41 to 44, and is biased on the left of FIGS. 11 and 12 by a compression coil spring 45 disposed in a fourth oil chamber 44. The first oil chamber 41 is always connected via a communication passage 46 to an upstream side of the metering orifice 36 provided in the transversal portion 35b of the discharge passage 35. The second oil chamber 42 is connected via communication passages 47 and 48 (see FIG. 10) to the suction opening 32 of the rear body 5.
A third oil chamber 43 is connected through a communication passage 50 to the upstream side of the metering orifice 36 in a state where the spool 38 is pressed by the compression coil spring 45 and abutted against a stopper 49 as shown in FIG. 11. The fourth oil chamber 44 is connected through a communication passage 51 to the downstream side of the metering orifice 36. Also, the fourth oil chamber 44 is connected via a relief valve 52 provided within the spool 38 to the second oil chamber 42, as shown in FIG. 9.
The valve bore 37 of the control valve 23 is connected through a first connecting passage 53 to the first fluid pressure chamber 21, and through a second connecting passage 54 to the second fluid pressure chamber 22, as shown in FIG. 9 opening positions of the connecting passages 53 and 54 on the side of the valve bore 37 are set such that the first connecting passage 53 is connected to the second oil chamber 42 and the second connecting passage 54 is connected to the third oil chamber 43 in a state where the spool 38 is abutted against the stopper 49, as shown in FIG. 11, or the first connecting passage 53 is connected to the first oil chamber 41 and the second connecting passage 54 is connected to the second oil chamber 42 in a state where the spool 38 is moved on the right, as shown in FIG. 12.
In the variable displacement pump 1 according to the related art having the above constitution, when the engine speed is in a range of low rotating speed including idling (range of A to B in FIG. 13), the spool 38 of the control valve 23 is pressed against the stopper 49 by a resilient force of the compression coil spring 45, as shown in FIG. 11. Because a pressure difference between the upstream side and the downstream side of the metering orifice 36 is small.
In this state, a pressure in the suction opening 32 is applied from the second oil chamber 42 of the control valve 23 to the first fluid pressure chamber 21, and a discharge pressure (an upstream pressure of the metering orifice 36) is applied from the third oil chamber 43 to the second fluid pressure chamber 22. Thereby, the cam ring 8 is held at a position as shown in FIG. 9, so that the pump volume of the pump chamber 11 formed between the rotor 3 and the cam ring 8 is at maximum and the discharge flow rate is also at maximum.
If the engine speed is increased and the flow rate of pressure oil passing through the discharge passage 35 is increased, there is a greater pressure difference between the upstream side and the downstream side of the metering orifice 36. Along with the increased pressure on the upstream side of the metering orifice 36, the pressure of the first oil chamber 41 in the control valve 23 is increased, so that the spool 38 is moved on the right against the resilient force of the compression coil spring 45, as shown in FIG. 12. Consequently, a discharge pressure is applied from the first oil chamber 41 to the first fluid pressure chamber 21, and a pressure of the suction opening 32 is applied from the second oil chamber 42 to the second fluid pressure chamber 22. Therefore, the cam ring 8 is swung on the right of FIG. 9 against a resilient force of the compression coil spring 17 of the urging means 15, decreasing the volume of the pump chamber 11 to make the discharge flow rate constant. During fast driving (C point in FIG. 13) where the cam ring 8 is swung up on the right end of FIG. 9, the discharge flow rate is constant at minimum.
The variable displacement pump 1 according to the related art having the above constitution has a problem that the energy loss amount is increased in a running state with great discharge flow rate, and it is found that this problem is caused by leakage of the pressure oil. That is, at the low rotating speed (in a range of A to B in FIG. 13), a pressure on the upstream side of the metering orifice 36 is introduced into the second fluid pressure chamber 22, and a high pressure oil supplied to the second fluid pressure chamber 22 at this low rotating speed is flowed through a small annular gap outside the adapter ring 9 into the first connecting passage 53 to leak into the second oil chamber 42 with lowest pressure within the control valve 23. By this amount of leakage, the pressure oil discharged from this variable displacement pump 1 is decreased. Hence, to make up for this amount of leakage, the engine speed must be increased to increase the discharge flow rate, resulting in the greater energy loss amount as previously described.
The small annular gap through which pressure oil is leaked may be composed of a first gap formed between the adapter ring 9 and the front body 4 and a second gap formed along the O-rings 12 and 13 attached to the rear body 5 and the pressure plate 7 to seal the pump chamber 11.
The first gap is formed when the adapter ring 9 or the front body 4 is deformed owing to a pressure oil acting on the outer circumferential face of the adapter ring 9. Into this gap, pressure oil of the second fluid pressure chamber 22 is leaked through the through hole 9a for the urging means 15 of the adapter ring 9 or an interstice formed between the rear body 5 and the pressure plate 7. To prevent pressure oil from leaking through the first gap, a structure is taken in which the cam ring is directly attached to the front body 4 without the use of the adapter ring 9. However, to adopt this structure, the front body 4 must be divided and formed at as high a precision as the adapter ring 9, increasing the costs remarkably.
On one hand, the second gap is formed when the O-rings 12 and 13 attached to the rear body 5 and the pressure plate 7 are pressed and compressed by a hydraulic pressure of the second fluid pressure chamber 22 to widen the space within the O-ring receiving portions 12a and 13a (see FIG. 10). To prevent pressure oil from leaking through the second gap, the fitting portion of the front body 4 and the rear body 5, the pressure plate 7 must be formed to make the gap as narrow as possible to prevent pressure oil from acting on the O-ring receiving portions 12a and 13a, resulting in the increased costs.
Also, in the variable displacement pump 1 according to the related art, a discharge pressure is always applied to the second fluid pressure chamber 22 during the period of low rotating speed, resulting in a problem that the pump body 2 must be formed securely and increased in size.
Thus, JP-A-2002-98060, which has been filed by Applicant, discloses a variable displacement pump, which can discharge pressure oil efficiently by preventing leakage of pressure oil from inside the pump while reducing the costs.
The variable displacement pump having a cam ring carried swingably inside an adapter ring, a first fluid pressure chamber provided in one of the swing directions of the cam ring, a second fluid pressure chamber provided in the other swing direction of the cam ring, urging means for urging the cam ring in a direction to maximize the volume of pump chamber, and a control valve for controlling the hydraulic pressure of the fluid pressure chambers on the both sides of the cam ring. The first and second fluid pressure chambers are connected to the control valve to be activated owing to a differential pressure between upstream side and downstream side of a metering orifice provided halfway on a discharge passage. The control valve is provided with a closing portion for closing a port connecting to the second fluid pressure chamber, when the differential pressure between the upstream side and the downstream side of the metering orifice is small.
The variable displacement pump has the advantages that it is possible to prevent pressure oil from leaking via the second fluid pressure chamber through the gap inside the pump, because no pressure oil is flowed into the second fluid pressure chamber at the low rotating speed. There is no need of increasing the size of the pump body for greater strength, because no discharge pressure is always applied on the second fluid pressure chamber.